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FOUS30 KWBC 021447
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1046 AM EDT SUN AUG 02 2015
...VALID 15Z SUN AUG 02 2015 - 12Z MON AUG 03 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NW TAD 25 WSW TAD 15 W RTN 15 W LVS 15 SE SAF 15 SSW SAF
20 NE AEG AEG 25 W AEG 10 ESE GNT GNT 35 NNW GNT 30 NE GUP
30 NNE RQE 50 N RQE 60 SW CEZ 55 S 4BL 40 SSW 4BL 40 SW 4BL
55 NE PGA 50 SSW 4HV 40 SSW 4HV 30 SW 4HV 15 SW 4HV 4HV 15 E 4HV
20 SW CNY 10 SSE CNY 15 E CNY 25 ENE CNY 20 WSW GJT 10 ENE GJT
10 SW RIL 10 W EGE 10 WNW CCU 30 SE LXV 40 SW FCS 30 NNE VTP
20 NW TAD.
SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
VVG 10 NW LAL 10 W SRQ 90 SSE AAF 10 SW 40J 45 SE VLD 20 SW VQQ
25 SE GNV VVG.
...FLORIDA...
DEEP TROPICAL MOISTURE RESIDING BENEATH A BROAD UPPER LEVEL TROUGH
WILL FEED SURGES OF CONVECTION THAT TRACK WEST TO EAST ACROSS THE
CENTRAL AND NORTHERN FLORIDA PENINSULA. THE GREATEST CONCERN IS
JUST TO THE SOUTH OF A SURFACE LOW PRESSURE CENTER...WHERE LOW
LEVEL CONVERGENCE AND LOW LEVEL INFLOW OFF THE GULF WILL BE
MAXIMIZED FOR QUITE SOME TIME...AS THE LOW MOVES VERY SLOWLY
EASTWARD. THIS INCLUDES THE STRETCH OF COAST FROM NEAR AND NORTH
OF TAMPA BAY UP THROUGH CEDAR KEY AND STEINHATCHEE. CONVECTION
HERE MAY BE SHALLOW AT TIMES...BUT STILL HIGHLY EFFICIENT AT
RAINFALL PRODUCTION GIVEN WARM RAIN PROCESSES AND TRAINING.
EFFICIENCY MAY BE ENHANCED DURING THE OVERNIGHT HOURS WHEN THE
BOUNDARY LAYER DECOUPLES AND STRONGER LOW LEVEL SOUTHWESTERLY
WINDS WILL CONTINUE TO PROGRESS ONSHORE. A BASIN AVERAGE OF 3 TO 4
INCHES OF QPF SHOULD BE EXPECTED BETWEEN KCTY AND KBKV.
IT SHOULD BE NOTED THAT SOME OF THE 00Z HIGH RESOLUTION
GUIDANCE...SUCH AS THE 00Z WRF ARW AND 00Z REGIONAL GEM...SHOW THE
POTENTIAL FOR AN ADDITIONAL 6 TO 9 INCHES OF RAINFALL WHERE
TRAINING OCCURS. THERE IS SOME MODEL SPREAD ON WHERE THE QPF
MAXIMUM OCCURS.
ELSEWHERE...THE VERY HIGH MOISTURE CONTENT ON THE LARGER SCALE
SHOULD SUPPORT LOCALLY INTENSE RAIN RATES THROUGHOUT FLORIDA...BUT
THERE ARE FACTORS COMBATING A MORE ORGANIZED LARGE SCALE EXCESSIVE
RAIN THREAT. LAPSE RATES HAD GROWN LESS STEEP WITH TIME AFTER A
FEW DAYS OF ACTIVE CONVECTION...AND LAPSE RATES WERE NOT BEING
REPLENISHED GIVEN RELATIVELY LITTLE LIFT ASSOCIATED WITH THE WEAK
TROUGH / SHEAR ZONE. DAYTIME CONVECTION WILL TEND TO PROPAGATE
WITH DECENT FORWARD MOTION ALONG OUTFLOWS...CUTTING DOWN ON
RESIDENCE TIME OVER A GIVEN POINT.
...COASTAL SOUTH CAROLINA AND GEORGIA...
A NARROW ZONE NEAR THE COAST MAY RECEIVE TWO ROUNDS OF HEAVY RAIN
THROUGH MONDAY MORNING. THUNDERSTORMS DEVELOPING DURING AFTERNOON
HEATING WILL MOVE VERY SLOWLY. LATER IN THE NIGHT SOUTHERLY LOW
LEVEL FLOW WILL INCREASE UP TO AND POSSIBLY ONTO THE COAST OF
SOUTH CAROLINA...AHEAD OF AN APPROACHING WEAK BUT WELL DEFINED LOW
LEVEL CYCLONE MOVING THROUGH NORTHERN FLORIDA. MODELS SUCH AS THE
00Z WRF-ARW AND WRF-NMM PLACE HEAVY RAINFALL ONSHORE IN SC LATE
TONIGHT...WHILE OTHERS REMAIN JUST OFFSHORE. QUESTIONS ABOUT
PLACEMENT AND RECENT TRENDS TOWARD POOR LAPSE RATES IN THE LOWER
TO MIDDLE LEVELS CAUSED WPC TO STOP SHORT OF ADDING A SLIGHT RISK
FOR THIS AREA...BUT RICH MOISTURE WILL SUPPORT ISOLATED VERY HEAVY
RAIN RATES WHEREVER CONVECTION DOES FORM.
...WESTERN UNITED STATES...
MONSOONAL MOISTURE IN THE SOUTHWEST STATES REMAINS CONNECTED TO A
PLUME EMANATING FROM THE EASTERN PACIFIC UP THROUGH SONORA.
PORTIONS OF THE MOISTURE ARE THEN DIVERTED INTO THE GREAT BASIN
AND SOUTHERN PLAINS...FOLLOWING A MID LEVEL SHEAR AXIS LAID OUT
ROUGHTLY WNW TO ESE. THE HIGH MOISTURE CONTENT AIR...COMBINED WITH
MARGINAL INSTABILITY AND LIFT FROM A SHORT WAVE CROSSING THE
SOUTHERN ROCKIES AND GREAT BASIN...IS EXPECTED TO PRODUCE AN AREA
OF SCATTERED TO BROKEN CONVECTION...WHICH COULD PRODUCE HEAVY TO
LOCALLY EXCESSIVE RAINFALL.
THE BEST THRUST OF MONSOONAL MOISTURE SHOULD OCCUR ACROSS NM INTO
THE SOUTHERN ROCKIES AND GREAT BASIN (AS SEEN ON THE LATEST HIGH
AND MID LEVEL LAYERED PRECIPITABLE WATER IMAGES). THERE IS GOOD
MODEL AGREEMENT THAT A SHORT WAVE TRACKS NORTH OUT OF AZ THROUGH
NV AND UT DURING THE AFTERNOON AND EVENING HOURS. THE SYNOPTIC
SCALE ASCENT ASSOCIATED WITH THE SHORT WAVE SHOULD ALLOW CLUSTERS
OF CONVECTION TO BLOSSOM ACROSS WESTERN CO (INCLUDING THE SAN JUAN
MOUNTAINS)...NORTHERN NM AND SOUTHEAST UT. PRECIPITABLE WATER
VALUES HERE SHOULD RANGE BETWEEN TWO AND THREE STANDARD DEVIATIONS
ABOVE THE MEAN...INCREASING THE THREAT FOR FLASH FLOODING. SOME OF
THE HIGH RESOLUTION GUIDANCE SUGGESTS LOCAL 2.00 INCH QPF AMOUNTS
ARE POSSIBLE WITH STORMS THAT TAP THE HIGH MOISTURE AIR...AND
BASED ON WHAT OCCURRED ACROSS WESTERN CO SAT...THIS SEEMS
POSSIBLE.
THERE IS SOME UNCERTAINTY CONCERNING HOW WIDESPREAD THE HEAVY RAIN
THREAT WILL BE...BUT WESTERN CO AND NORTHERN NM SEEM TO BE IN THE
AREAS WITH THE BEST COMBINATION OF LIFT/MOISTURE. WHILE BASIN
AVERAGE QPF AMOUNTS BETWEEN 0.25 AND 0.50 INCHES MAY BE COMMON IN
THE HIGHER TERRAIN...HIGHER AMOUNTS IN AREAS WHERE ONE AND THREE
HOUR FLASH FLOOD GUIDANCE VALUES ARE AN INCH OR LESS WILL CAUSE
PROBLEMS. A SLIGHT RISK AREA WAS ISSUED FOR THIS AREA.
FARTHER WEST...INSTABILITY AND MOISTURE COULD SUPPORT SCATTERED
CONVECTION WITH LOCALLY HEAVY RAINFALL ACROSS PORTIONS OF NV INTO
NORTHERN CA AND WESTERN OR. PRECIPITABLE WATER VALUES FROM
NORTHERN CA/SOUTHWEST OR THROUGH MUCH OF NV SHOULD TOP OUT BETWEEN
TWO AND THREE STANDARD DEVIATIONS ABOVE THE MEAN....AND MARGINAL
INSTABILITY SHOULD INITIATE CONVECTION MAINLY OVER THE TERRAIN.
SOME OF THE MORE AGGRESSIVE HIGH RESOLUTION GUIDANCE (LIKE THE 00Z
NAM CONEST) SUGGESTS THAT LOCAL 1.00+ INCH AMOUNTS ARE POSSIBLE
ACROSS THE TERRAIN OF CENTRAL NV. WITH A SHORT WAVE CROSSING THE
REGION FOR SYNOPTIC SCALE ASCENT...LOCAL QPF AMOUNTS OF THIS
MAGNITUDE ARE PLAUSIBLE. GIVEN THE HIGH MOISTURE CONTENT OF THE
AIR...A SEE TEXT WAS PLACED OVER NV.
HAYES/BURKE
$$